1. Field of the Invention
The invention relates to a control apparatus for a hybrid vehicle that is equipped with two types of drive sources, namely, an engine and a motor.
2. Description of Related Art
Hybrid vehicles that are each equipped with two types of drive sources, namely, an engine and a motor have been put into practical use. Some of such hybrid vehicles are known to make a changeover among a plurality of driving modes. For example, in each of Japanese Patent Application Publication No. 2009-149161 (JP-2009-149161 A), Japanese Patent Application Publication No. 2009-126450 (JP-2009-126450 A), Japanese Patent Application Publication No. 2009-143315 (JP-2009-143315 A), and Japanese Patent Application Publication No. 2009-126253 (JP-2009-126253 A), there is described a hybrid vehicle that can make a changeover among three driving modes, namely, a normal driving mode, a power driving mode, and an economy driving mode.
FIG. 4 shows a relationship between an accelerator depression amount and a required output value in each driving mode in a common hybrid vehicle. As shown in FIG. 4, in the power driving mode, the required output value for the same accelerator depression amount is set larger than in the normal driving mode, and higher priority is given to the responsiveness of a driving force than in the normal driving mode. On the other hand, in the economy driving mode, the required output value for the same accelerator depression amount is set smaller than in the normal driving mode, and higher priority is given to an improvement in fuel economy than in the normal driving mode.
Besides, it is described in each of Japanese Patent Application Publication No. 2009-149161 (JP-2009-149161 A), Japanese Patent Application Publication No. 2009-126450 (JP-2009-126450 A), and Japanese Patent Application Publication No. 2009-143315 (JP-2009-143315 A) that the responsiveness of a driving force is enhanced by enlarging a range where the intermittent operation of an engine is prohibited, at the time of the power driving mode. Further, it is described in Japanese Patent Application Publication No. 2009-126253 (JP-2009-126253 A) that when the power driving mode is selected at a low vehicle speed, the engine is autonomously operated to attempt to restrain the responsiveness to a request for acceleration and the state of charge (the SOC) of a battery from falling.
By the way, in such a hybrid vehicle, when the SOC of the battery falls, the battery is charged with an electric power generated through a power of the engine, so as to attempt to recover the fallen SOC. However, if the engine is operated for electric power generation when the vehicle speed is low, vibrations of an exhaust pipe resulting from vibrations of exhaust gas discharged from the engine cause noise. Thus, in the hybrid vehicle, when the vehicle is stopped or runs at a low vehicle speed, the charge amount of the battery is regulated to restrain the engine from being operated to generate an electric power.
However, when the charge amount is limited, the recovery of the SOC is delayed. Besides, when the SOC is deficient, the driving force cannot be sufficiently supplemented by the motor, and therefore, it may become impossible to ensure high power performance required at the time of the power driving mode.